The present invention relates to a non-volatile semiconductor memory capable of writing at a low voltage and a low current.
A conventional channel injection-type non-volatile semiconductor memory is disadvantageous in writing efficiency and requires a large current and large voltage for writing. The reason for the above disadvantage will be illustrated in conjunction with a sectional view of a conventional channel injection-type non-volatile semiconductor memory shown in FIG. 1.
In FIG. 1, a P-type semiconductor substrate 5 comprises an N.sup.+ type source region 3 and drain region 4 near the surface, an insulator layer 7 such as a silicon oxidation layer or nitride layer formed on a channel region between the source 3 and the drain 4, a floating gate electrode 2 for storing electric charges as information formed on the insulator layer 7, an insulator layer 6 formed on the floating gate electrode 2, and a control gate electrode 1 formed on the insulator layer 6. In the N-channel type non-volatile semiconductor memory, when a voltage VCG which is positive relative to the substrate 5 is applied to the control gate electrode 1 to invert the surface of the substrate 5, and a positive voltage is applied to the drain region 4, negatively charged electrons e.sup.- are accelerated near the drain region 4 as shown in FIG. 1. And when the electrons acquire sufficient energy to pass over the electric potential barrier of the insulator 7, the electrons can be injected into the floating gate electrode 2. As the electrons approach the drain, fewer are injected into the floating gate electrode 2 and more remain inside the semiconductor. The electrons injected into the floating gate electrode 2 are a part of the high energy electrons produced by collisons of the accelerated electrons inside the electric field of the depletion layer extended by the drain voltage VD near the drain with silicon atoms inside the silicon crystal. Among the high energy electrons produced by the collisions, only the electrons scattered in the direction of the floating gate electrode 2 can be injected into the floating gate electrode 2. The other high energy electrons flow into the drain region 4 that has a low energy level relative to the electrons. Namely, an exceedingly small number of electrons are injected into the floating gate electrode 2 out of the total channel current flowing from the source region 3. The rate of the injected electrons is generally 10.sup.-10 -10.sup.-8. Accordingly, to compensate for the low injection efficiency, the conventional non-volatile semiconductor memory as shown in FIG. 1 has injected the electrons necessary for acting as memory by the following methods:
(1) The channel current has been enlarged to increase injection electrons.
(2) The drain voltage has been raised to increase the electric field of the depletion layer near the drain and to increase the impact ionization.
(3) The injecting period has been prolonged to inject the necessary electrons into the floating gate electrode 2.
As understood from the above (1)-(3) methods, however, it is difficult to improve the integration degree of the memory and to write at high speed since the conventional memory necessitates a large current, a large voltage and a long time to write information on the memory.